COURSE 4 PROJECTS

QUANTUM FLUCTUATIONS IN SUPERCONDUCTING NIOBIUM

Supervisor: Dr. Elizabeth Blackburn
Room: Physics East 207
Tel: 44694
e-mail: e.blackburn@bham.ac.uk

Summary

Superconductivity is a great example of large-scale quantum mechanical effects. One of the defining features of this is that a superconductor expels magnetic field (the Meissner effect). When some superconductors are put in a sufficiently large magnetic field, the magnetic field entering the superconductor as quantised flux lines [1]. These flux lines order into a 2D lattice, and such lattices have been well studied here at Birmingham [2]. We investigate these materials using a variety of methods, and projects can be adapted to the interests of the project students. One project is described in detail here, and another is briefly described at the end.

Project Description

We can imagine thermal or quantum fluctuations causing this flux line lattice to melt into a liquid state, and such a state has been the focus of much experimental and theoretical interest [3]. This project is an opportunity to investigate this, using low temperature resistivity and/or heat capacity measurements to look for these effects.

Background

Niobium is a superconductor with a transition temperature ~ 9 K, and we have prepared ultra-pure single crystals, allowing measurements of its intrinsic properties and making it an ideal material in which to investigate this flux line lattice melting. However, the proposed liquid state only exists over the small range of fields and temperatures where fluctuations are causing superconductivity to disappear. We have carefully designed heat capacity and resistivity measurement rigs with an excellent temperature resolution < 100 μK to investigate. To date, we have seen no sign of a sharp peak in the heat capacity corresponding to a melting transition [3], but we have seen a broad peak indicating that the fluctuations in the superconductor have changed as the flux line lattice disappears. There will be opportunities to improve the experimental setup, for those interested in hands-on electronics. We propose to investigate this region as a function of temperature down to T ~ 50 mK, involving use of specialized cryogenic equipment, as we suspect that the fluctuations will become predominantly quantum in nature near to absolute zero.

Alternative projects

(i) To study the flux line lattice directly, it can be seen by diffraction of neutrons. We have recently tried some new methods for collecting the data, using a large spread of wavelengths as opposed to rotating the sample to examine the diffraction spots, but this requires some development to extract the fundamental properties of the superconductor.

(ii) In a liquid crystal, transitions between different phases can be driven by electric fields, as in LCD screens. They can also be affected by magnetic fields. We have started some studies of these effects, and this project would involve setting up a rig for optical measurements of a liquid crystal sample in an electromagnet, to investigate the Frederiks transition.

(iii) We have some apparatus to measure the resistivity of metal samples over a range of temperatures down to liquid helium temperatures. We need to test some high purity aluminium for use in future experiments, and can use this apparatus to judge the impurity level. This project would involve changing the computer control system for the experimental rig, and testing the aluminium.